Right then ... it is a bit complicated and I may well have lost track through the ailments. Started before Christmas when he had a hyperactive thyroid - this was removed, took him home and he fitted. Took him back and apparently the parathyroid was damaged, so he was put on the Alpha-One drops until he stabilised (think these are for people, but very low dose). Everything was fine for a while.
About about six weeks ago he fitted again (twice) and then a third time which was when eilepsy was diagnosed. He is now on daily anticonvulsants (same as humans and luckily cheap).
Then... about two weeks ago he stopped eating, moving, etc and I really thought he was a goner. In desperation and as there was nothing to lose, I started him on the drops again and he made a miraculous recovery. Have spoken to the vet and he reckons there is no problem with keeping him on the drops (he is due back for a check next week). Problem is that I don't really want to go through the blood test routine again, largely because of the cost. Love him to bits, but up to £1200 and counting and the bank manager will complain soon.
No signs of renal problems. Maybe I should ask the vet about VitD3 and see what he has to say?